In Java(tm), null is not a keyword, but a special literal of the null type. It can be cast to any reference type, but not to any primitive type such as int or boolean. The null literal doesn't necessarily have value zero.
Add a jump rope to the motion and you will be skipping rope. The skipping moves won't bulk you up, but they can tone the muscles in your thighs, calves and butt -- your quadriceps, hamstrings, gluteals, iliopsoas, gastrocnemius and tibialis anterior.
1 Your musclesâthe gastrocnemius, quadriceps, hamstrings, glutes, abdominals, forearms, and deltoidsâget a workout. [ 2 sidebar]Your agility, hand-to-eye coordination, and balance will improve so that you'll feel more sure on your feet.  Rope jumping is also second to none for building strong bones.